[9.02] IZw18 has a Dynamically Associated Companion Galaxy

R. J. Dufour (RiceU), H. O. Castañeda (VILSPA), C. Esteban (IAC)

We present kinematical evidence that the irregular galaxy located 26''
northwest of IZw18 -the most metal-poor blue-compact-dwarf (BCD)
galaxy known- is a dynamically associated companion system.
Longslit CCD spectra were obtained in 1996 February using the 4.2m WHT+ISIS
at La Palma, with the slit placed across the NW HII region
of IZw18 and through
an H\alpha knot in the center of the companion galaxy.

Deep CCD spectra were acquired simultaneously in the blue and red spectral
regions; with the blue covering H\beta and
[OIII] 4959,5007Åand the red covering H\alpha. The red spectra showed
continuous H\alpha emission from the BCD main body of IZw18
to the H\alpha knot in
the companion galaxy. The heliocentric radial velocity variation for
the H\alpha line across
a 50'' length of slit
(\sim2.4 kpc for a distance of 10 Mpc for IZw18) shows a smooth
double-sinusoidal variation ranging from +730 km/s just NW of the brightest
star-forming region in IZw18 to +780 km/s in the SE extremity of
the main body. The heliocentric velocity of the brightest H\alpha knot
in the main body was measured as +741.0\pm0.1 km/s and that of the H\alpha
knot in the companion was found to be +752\pm2 km/s (where the errors
are the residuals of the gaussian fits; systematic errors are
yet to be evaluated).

In addition to the radial velocity information, we present an analysis of
high velocity gas seen in the wings of H\alpha and other lines at several
locations across IZw18 and in the H\alpha knot of the companion
system (where we find the knot to be an expanding cloud,
with v_exp\sim100 km/s).
Previous HST WFPC2 imagery (Dufour et al. 1995, BAAS,
27, 86) indicated that the companion system (``C'') was a
dwarf irregular galaxy of type Im that
resolved into stars at the V\approx24.5 level. They also noted that
it contained
stars as young as \sim40 Myr -if it were at the same distance as IZw18.
Our new results prove that ``IZw18'' consists of a pair of
dwarf irregulars, one
currently undergoing a starburst (the BCD namesake) and another
nearby (\sim1.5 kpc in the plane of the sky) Im system which
has had an older star formation history.